74 FIRST FORMS OF VEGETATION. 
ing, or the plant thrown on the soil at the margin, 
they become smaller, and fasten themselves firmly 
to the ground by numerous fibrous rootlets,—a 
beautiful example of the ease with which these 
humble plants accommodate themselves to altered 
circumstances. They have many air-passages 
between the cells, which enable them to float on 
the water. The under surface is covered, to a 
greater or less extent, with thin scales, which form 
most beautiful microscopic objects when treated 
with different chemical tests, from their transpa- 
rency and variety of colouring. One ally of this 
genus, called Riella, differs widely from the rest 
of the tribe in its erect, moss-like habit. It grows 
on the margins of ponds, streams, and lakes in 
Algiers and Sardinia, and perfects its fruit when 
submerged. It is quite a botanical curiosity, pre- 
senting a whorled appearance, not unlike the 
common spiral shells of the sea-shore. Each in- 
dividual consists of a central stem, round which a 
distinct leaf or wing is wound in the form of a 
screw or continuous spiral. On the edge of this 
wing, towards the summit of the male plant, the 
antheridia are developed ; while in the female the 
fruit clusters on the stem between the whorls. 
An example of this beautiful genus is very com- 
mon on moist garden paths and on the mould of 
pots in the green-house and stove. It forms little 
